Semiconductor device fabrication involves performing a variety of processes, procedures and operations in order to achieve a fabricated device. These operations include, but are not limited to, layering, doping, heat treatments, and patterning. Layering is the operation used to add layers of a selected thickness to a wafer surface. These layers can be insulators, semiconductors, conductors, and the like and can be grown or deposited by a number of suitable methods (e.g., chemical vapor deposition, sputtering, and the like).
Doping is the process that introduces specific amounts of dopants in the wafer surface through openings in surface layers. Two general techniques of doping are thermal diffusion and ion implantation. Doping is used, for example, to create active regions in transistors.
Heat treatments are operations in which a wafer is heated and cooled to achieve specific results. Generally, no additional material is added although contaminates and vapors may evaporate from the wafer surface. A common heat treatment is called an anneal which is typically employed to repair damage to crystal structures introduced by ion implantation.
Patterning is the operation that employs a series of steps that results in the removal of selected portions of added surface layers. The series of steps includes first forming a layer of resist or photoresist over a semiconductor device. Then, a resist mask or reticle is aligned with the device. Subsequently, the layer of resist is exposed or irradiated through the resist mask, which selects portions of the layer of resist that are later removed to expose underlying portions of the device. Continuing, a fabrication process, such as ion implantation, ion diffusion, deposition, etching, and the like is performed on exposed portions of the device.
Semiconductor devices typically include transistor devices, such as MOS and CMOS devices, and resistors. Many transistor devices have gate electrodes formed with polysilicon material. Similarly, many resistors are also formed with polysilicon material and are, therefore, referred to as polysilicon resistors or poly resistors. The polysilicon gate electrodes are doped with a particular dopant type and amount to obtain desired transistor behavior. The polysilicon resistors are doped with a particular dopant type and amount to obtain a selected contact resistance and overall resistance. However, dopant types and amounts employed for the gate electrodes are generally different than the dopant types and amounts employed for the polysilicon resistors. Furthermore, polysilicon resistors on a semiconductor device can be designed with differing contact resistance and resistance thereby requiring varied dopant types and/or dopant concentrations. Unfortunately, obtaining these varied dopant types and/or amounts requires discrete doping processes, including ion implantation and diffusion, which typically require separate masks and patterning operations. As a result, a substantial number of extra processing steps are typically employed to fully form the polysilicon gates and the polysilicon resistors because of the varied dopant types and/or amounts employed for each.